1. Field of the Invention 
The present invention relates to an air flow rate measuring apparatus for outputting a signal in response to a flow rate of a fluid and particularly to an air flow rate measuring apparatus suitable for measuring an intake air flow rate in an internal combustion engine. 
2. Description of the Related Art 
Improvements in measurement accuracy in low flow rate regions and improvements in measurement accuracy during transitions are required in conventional air flow rate measuring apparatuses used to measure the flow rate of intake air in internal combustion engines due to the expansion of measurement ranges for intake air flow rate accompanying increased output in internal combustion engines and also due to the tightening of emission regulations. 
In conventional air flow rate measuring apparatuses, a diversion passage having an inverted U shape in which two adjacent, parallel channels are linked by a curved portion is installed inside an airflow channel. Thus, the overall length of the diversion passage is increased, reducing surges inside the diversion passage resulting from surges in the flow of air inside the airflow channel. In addition, the curved portion of the diversion passage and a downstream passage of the diversion passage are formed into a shape that suppresses contracted flow arising downstream from the curved portion. Thus, flow rate measuring precision is improved by suppressing decreases in flow velocity due to contracted flow to maintain flow velocity inside the diversion passage even in the low flow rate regions. 
However, in conventional air flow rate measuring apparatuses, since side walls facing each other in a direction perpendicular to the direction of a main flow are present at an inflow port into the diversion passage, a portion of the flow having an angle inclined) relative to the parallel flow of the main flow is obstructed by the side walls and is prevented from flowing into the diversion passage. Thus, increased flow rate sensitivity due to increased flow velocity inside the diversion passage becomes minimal. Consequently, the flow velocity inside the diversion passage is slow, leading to a decrease in the flow rate sensitivity and making output of the flow rate signal unstable. 
Since the side walls facing each other in the direction perpendicular to the direction of the main flow have a predetermined thickness in a direction perpendicular to the direction of flow of the main flow, the flow of the main flow colliding with the side walls at right angles is interrupted temporarily by the side walls, giving rise to a delay before flowing into the diversion passage. Consequently, the state of the flow into the diversion passage is unstable, making output of the flow rate signal unstable. 
In air flow rate measuring apparatuses for internal combustion engines, foreign matter such as droplets of liquid (water, oil, etc.), dust etc., contained in the air flowing through the airflow channel is blown and introduced into the diversion passage from the upstream end. When this foreign matter such as droplets of liquid, dust, etc., adheres to inner wall surfaces of the diversion passage or the flow rate detecting element, there is a risk that the flow velocity distribution inside the diversion passage, and the heat-transfer coefficient of the surface of the flow rate detecting element, etc., may change, giving rise to changes in the output of the flow  rate signal. In conventional air flow rate measuring apparatuses, because the side walls facing each other in the direction perpendicular to the direction of a main flow are present at the inflow port into the diversion passage and the flow velocity inside the diversion passage is slow, foreign matter that has been introduced into the diversion passage and has adhered to the inner wall surfaces of the diversion passage is unlikely to be blown back out through an outflow port by the flow inside the diversion passage and most of it remains inside the diversion passage. Consequently, output fluctuations in the flow rate signal may become large depending on the location and amount of adhesion of foreign matter, preventing accurate flow rate detection. 